The U.S. Transportation Security Administration (TSA) and other similar agencies throughout the world are responsible for identifying dangerous devices and/or contraband within passenger baggage brought onboard public transportation systems. Various X-ray baggage scanning systems are known for detecting the presence of such materials, e.g., explosives and other prohibited items, in baggage or luggage prior to loading the baggage onto commercial carriers, e.g., a commercial aircraft. Since many explosive materials may be characterized by a range of densities differentiable from that of other items typically found in baggage, explosives are generally amenable to detection by X-ray equipment. A common technique of measuring the density of a material is to expose the material to X-rays and to measure the amount of attenuation, and therefore radiation absorbed by the material, the absorption being dependent on the density and atomic number of the material.
Many X-ray baggage scanning systems in use today are of the “line scanner” type and include a stationary X-ray source, a stationary linear detector array, and a conveyor belt for transporting baggage between the source and detector array as the baggage passes through the scanner. The X-ray source generates a stationary X-ray beam that passes through and is partially attenuated by the baggage, as the baggage is moved into and positioned within the beam, before being received by the detector array. During each measuring interval, each detector of the detector array generates data representative of the integral of the density of the segment of the baggage through which the detected portion of the X-ray beam passes. The data acquired by the detector array during each measuring interval is used to form one or more raster lines of a two dimensional image. As the conveyor belt transports the baggage past the stationary source and detector array, the scanner generates a two dimensional image closely related to the areal density of the baggage, as viewed by the stationary detector array. The image is typically displayed for analysis by a human operator.
Recently developed baggage scanners have employed X-ray computed tomography (CT) to improve the image by eliminating the line integral and producing images that have pixel values related to physical density instead of areal density. These CT scanner images are then used to identify objects within baggage positioned in the scanner. Representative examples include the AN6400 EXACT system manufactured by the current assignee and the Invision CTX 9000® system manufactured by GE Invision, Inc. Such systems typically include a CT scanner that includes an X-ray source and an X-ray detector system secured to diametrically opposite sides of an annular-shaped platform or disk. The disk is rotatably mounted within a supporting frame, or gantry support, so that in operation the disk can rotate while X-rays pass from the source through an object positioned within the opening of the disk to the detector system.
The detector system can include a linear array of detectors disposed as a single row in the shape of a circular arc having a center of curvature at the focal spot of the X-ray source, i.e., the point within the X-ray source from which the X-rays emanate. The X-ray source typically generates a fan-shaped beam, or “fan” beam, of X-rays that passes through a planar imaging field, and is received by the detectors. More advanced CT scanners include an array of detectors, arranged in multiple rows, which are simultaneously irradiated by an X-ray source “cone” beam so that data relating to multiple slices can be simultaneously acquired.
CT scanners are typically characterized by a rectilinear coordinate system, e.g., one defined by X-, Y- and Z-axes, in which the axes intersect and are all normal to one another at the center of rotation of the disk during its operational motion. This center of rotation is commonly referred to as the “isocenter.” The Z-axis is defined by the rotation axis and the X- and Y-axes are defined by and lie within the imaging field. The fan and cone beams are thus each defined as the volume of space defined between the X-ray source and the receiving surfaces of the detectors of the detector array that are exposed to the X-ray beam emanating from the source. Each detector generates an output signal representative of the intensity of the X-rays incident on that detector.
As the disk rotates around the object being scanned, the scanner generates a plurality of projections at a corresponding plurality of projection angles. Since the X-rays are partially attenuated by all the mass in their path, the output signal generated by each detector is representative of the density of the portion of any object disposed in the imaging field and in the projection direction between the X-ray source and that detector.
A CT image of the object may be generated from all the projection data collected at each of the projection angles by using appropriate reconstruction software and algorithms. In a multi-slice scanner the CT images are representative of the density of a plurality of two dimensional “slices” of the object through which the cone beam has passed during the rotation of the disk through the various projection angles. The data from multiple slices can be used to reconstruct and display three dimensional images of the scanned object.
Further, the CT type baggage scanners use a conveyor system including a conveyor belt to move objects into the scanner one object at a time. In the reference Invision machine the conveyor stops when an object is positioned in the scanner, and a constant axis CT scan (with no movement in the Z-axis direction) is performed on the object. In order to increase throughput, the AN6400 EXACT baggage scanner is designed to move the objects continuously through the CT scanner while helical scans (where there is movement in the Z-axis direction during a scan) are performed.
Since X-rays can be harmful, shielding of some sort is typically used with X-ray scanners to protect people in proximity to the scanners. Government regulations may require such shielding to reduce detectable levels of radiation outside of the scanners to certain levels. Often high density materials such as lead or bismuth are used to provide such shielding. In the Invision scanner, for example, doors of shielding material are provided above the conveyor at the entrance and exit ports of the scanner. The doors are designed to open sequentially when objects are conveyed on the conveyor belt in and out of the scanner, so that whenever x-rays are on, at least one door on each side of the x-ray beam is closed. In the AN6400 EXACT scanner curtains of flexible strips of X-ray absorbent materials are provided at the entrance and exit ports of the scanner so as to provide shielding since the belt continuously moves even when scans are being performed.
For X-ray inspection systems that use such movable shield doors, the shields doors have presented problems including frequent break downs due to the complexity of the associated door movement mechanisms. This can have a negative impact on the throughput of articles passing through the related X-ray inspection systems, something that is undesirable for busy public transportation systems such as international airports.
Curtains with hanging strips of high-X-ray absorption fabric such as provided on the AN6400 EXACT scanner, allow for higher throughputs when used with a continuous running conveyor and a helical CT scanning inspection station. Such curtains each typically include a row of strips aligned side-by-side (contiguous with one another) across the width of the tunnel. The strips hang in the same vertical plane within the tunnel, most often just above or in contact with an associated conveyor belt. The strips are able to bend and flex to a degree to allow passage of articles through the tunnel by the conveyor system. These types of shield curtains are typically located at the entrance and exit ports of a containment tunnel. The curtains may also be located at one or more locations along the tunnel between the entrance and exit ports to provide improved shielding since at any one time, the strips of one or more curtains may be displaced by a moving bag.
Such prior art curtains may be effective when used with a continuously operating X-ray source and when overall article throughput is low and the total radiation in the tunnel is limited by either low x-ray beam current or high degree of x-ray collimation. Despite this, the use of such prior art curtains still present certain problems. For example, when the collimated X-ray beam is expanded beyond a certain size, or when the flux within a system is increased above a certain level, the radiation that escapes through the curtains at the ports as the strips are displaced by objects passing through can increase above acceptable levels. Because the X-ray source is continually operating, unacceptable radiation levels can escape unimpeded from a tunnel when the strips in such curtains are displaced by a moving bag. Additionally, because the individual strips of such curtains tend to curl about the vertical axis as they are used over a period of time, the effectiveness of these types of curtains can degrade to an unacceptable degree over time.
Utilizing thicker curtains may not be a viable solution under all circumstances. Increasing the thickness of the fabric decreases the flexibility of the curtain materials. As a result, the strips of the curtains are less able to be displaced and/or bend when contacted by articles to be inspected. The articles can consequently experience increased resistance against movement within the inspection system. This can cause articles to jam in the inspection system, compromising its effectiveness.
It would be beneficial, therefore, for providing adequate radiation attenuation shielding that allows for use of increased X-ray fluxes and beam diameters in X-ray inspection systems, whether continuously or intermittently operating, while achieving given radiation levels outside of the systems. At the same time, it would be beneficial for radiation attenuation shielding that allows X-rays inspection systems to better accommodate increased volumes or throughput of articles such as passenger baggage. It would also be beneficial for the radiation attenuation shielding to accommodate a wide range of sizes of articles such as those of common passenger baggage.